This invention relates generally to industrial lift trucks and in particular to an overload warning system which signals the operator when an excessive fork load has been encountered. Additionally, this invention relates to an improvement in the vehicle disclosed and claimed in U.S. Pat. No. 3,207,249 issued Sept. 21, 1965, under the title Power Plant Support Means for Industrial Vehicle.
The load capacity of an industrial lift truck is determined by several factors. In the type of lift truck on which the lifting apparatus is mounted forward of the front wheels, the load capacity will be primarily determined by the weight of the vehicle to the rear of the front wheels and the location of its center of gravity. In operation, the load being elevated creates a moment force about the axis of the front wheels tending to tilt the vehicle forward. The weight on the rear of the vehicle acting as a "counterweight" creates an opposing downward moment tending to right the vehicle. As long as the moment force created by the "counterweight" exceeds the moment force established by the lift load, the vehicle will remain stable. Essentially, the front wheels act as a fulcrum balancing the "counterweight" and load forces.
Even though a very significant portion of a vehicle's weight is constant, the weight of the rear of the vehicle and the location of its center of gravity will vary. Examples of factors which cause the weight to vary are: the weight of the operator, the weight of non-standard equipment and accessories, and the weight of fuel being carried. During the course of operation, the load capacity of the vehicle will fluctuate as these and other parameters change.
The magnitude of the load created moment force is determined by the weight of the load and the distance of its center of gravity from the front wheels. Only the adverse combination of both of these parameters can define and precipitate an unstable condition; the value of only one parameter cannot. It should be readily apparent that a given "stable" load condition may become an unstable condition if the center of gravity of the load is far enough from the front wheels to establish a moment force greater than that created by the rear "counterweight". Further compounding the problem is the fact that as the lift mast is tilted forward the effective moment arm between the front wheel axis and the load center of gravity increases.
Several prior proposals have been suggested which, directly or indirectly, sense a load being carried on the lift mast. One proposal would use pressure sensitive transducers in the hydraulic lifting circuit. The pressure in the mast lift cylinder is related to the weight of the load, but the pressure generated by given load will vary depending on the tilt angle of the mast. As it is tilted forward, the force generated by the load will be distributed between the tilt and the lift cylinders. It would seem that for a given load weight, this proposed system would see less load as the mast tilt angle increased even though increasing the tilt angle in fact increases the potential for an unstable condition. A further disadvantage of this system is its insensitivity to the load center of gravity. As was discussed earlier, it is the moment force established by the load that precipitates an unstable condition and not the load weight alone. This proposed system would not recognize a load capable of generating an unstable condition due to its center of gravity location.
Another proposal would utilize a strain guage mounted to the tilt cylinder hardware, specifically the clevis pin. Generally, the force applied to the tilt cylinder during operation will be related to the moment force applied by the load. The forces on the cylinder are not however, related to the stabilizing moment force established by the weight of the rear of the vehicle. This proposal is insensitive to fluctuations in the rear "counterweight" forces. Additionally, sophisticated and costly processing equipment is necessary to deal with the strain guage signal.
A third proposal relies on the fact that the load being carried tends to raise the rear portion of the vehicle, unloading the rear suspension. The rear suspension loading is related to the moment force applied by the lift mast carried load, and to the moment force applied by the weight on the rear of the vehicle. In fact, the rear suspension loading is directly related to the net difference between the two opposing moment forces. The unloading of the suspension is physically manifested as an increased distance between the unsprung and sprung portions of the rear of the vehicle.
Prior proposals would utilize movement due to unloading the rear suspension to actuate switches placed on various support members and at various locations. One such proposed system suggests the placement of a switch above the rear spring support pivot of a leaf spring type suspension in an industrial lift truck. A housing surrounding the spring support allows it to move vertically a limited amount, in response to the load being carried on the front of the vehicle. A switch is mounted above the support and is actuated by an associated plunger upon a predetermined movement in this rear spring support. Since in typical leaf spring suspensions, the spring mounting points are rigidly secured to the vehicle frame, the mounting apparatus required to accomplish this proposed system represents an added complexity in vehicle suspensions.
Other such systems suggest the placement of actuating members intermediate the ends of the leaf spring such that a predetermined movement in the leaf spring actuates a switch mounted above the actuating member. This system, among other shortcomings, is affected adversely by spring fatigue.
Because all of the systems would employ only suspension activated switches, these systems would not correlate the vehicle mast height with the rear suspension movement and therefore these proposals would be sensitive to only one unstable condition. As was discussed the vehicle load capacity decreases with the distance to which the load is raised and therefore correlation between mast height and suspension loading is an important consideration.
Generally, these prior proposed suspension activated overload systems would employ only one switch mounted to one side of the vehicle. The sensing of suspension load on only one side of the vehicle, may result in erroneous load indications when an eccentric load is being maneuvered. Depending on suspension configuration systems employing single suspension activated switches, may not reliably respond to an overload in which the center of gravity is near the vehicle side to which a sensing switch is mounted. Thus, the presence of a potential unstable condition would not be signalled to the operator.
Mounting and adjustment of these proposed suspension actuated switches would be critical and being mechanical in nature would tend to require periodic readjustment. Additionally, the mechanical suspension movements on which these systems would rely are subject to frictional and hysteresis effects.